GammaIInv(Y, A, B)

Returns the inverse of the gamma function. The function returns the value x such that

GammaI(x, a, b) = y.b

A is the shape parameter, B is an optional scale factor (default B=1). Note that some textbooks use [math]\displaystyle{ \lambda = 1/A }[/math]as the scale factor. The incomplete gamma function is defined for [math]\displaystyle{ X\geq 0 }[/math].

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